exec() an locals() puzzle

[george trojan]

Thanks. That cdef-locals concept is consistent with the following example:

def f():
    i = 1
    def g(): print('i' in globals(), 'i' in locals())
    def h(): print('i' in globals(), 'i' in locals()); i
    g()
    h()
f()

False False
False True

It is a mystery, which may be why the documentation for globals() and
locals() is 2-line long.


Le mer. 20 juill. 2022, à 19 h 31, Martin Di Paola <
martinp.dipaola at gmail.com> a écrit :

> I did a few tests
>
> # test 1
> def f():
>      i = 1
>      print(locals())
>      exec('y = i; print(y); print(locals())')
>      print(locals())
>      a = eval('y')
>      print(locals())
>      u = a
>      print(u)
> f()
>
> {'i': 1}
> 1
> {'i': 1, 'y': 1}
> {'i': 1, 'y': 1}
> {'i': 1, 'y': 1, 'a': 1}
> 1
>
> # test 2
> def f():
>      i = 1
>      print(locals())
>      exec('y = i; print(y); print(locals())')
>      print(locals())
>      a = eval('y')
>      print(locals())
>      y = a
>      print(y)
> f()
> {'i': 1}
> 1
> {'i': 1, 'y': 1}
> {'i': 1}
> Traceback (most recent call last):
> NameError: name 'y' is not defined
>
>
> So test 1 and 2 are the same except that the variable 'y' is not
> present/present in the f's code.
>
> When it is not present, exec() modifies the f's locals and adds an 'y'
> to it but when the variable 'y' is present in the code (even if not
> present in the locals()), exec() does not add any 'y' (and the next
> eval() then fails)
>
> The interesting part is that if the 'y' variable is in the f's code
> *and* it is defined in the f's locals, no error occur but once again the
> exec() does not modify f's locals:
>
> # test 3
> def f():
>      i = 1
>      y = 42
>      print(locals())
>      exec('y = i; print(y); print(locals())')
>      print(locals())
>      a = eval('y')
>      print(locals())
>      y = a
>      print(y)
> f()
> {'i': 1, 'y': 42}
> 1
> {'i': 1, 'y': 1}
> {'i': 1, 'y': 42}
> {'i': 1, 'y': 42, 'a': 42}
> 42
>
> Why does this happen? No idea.
>
> I may be related with this:
>
> # test 4
> def f():
>      i = 1
>      print(locals())
>      exec('y = i; print(y); print(locals())')
>      print(locals())
>      print(y)
> f()
> Traceback (most recent call last):
> NameError: name 'y' is not defined
>
> Despite exec() adds the 'y' variable to f's locals, the variable is not
> accessible/visible from f's code.
>
> So, a few observations (by no means this is how the vm works):
>
> 1) each function has a set of variables defined by the code (let's call
> this "code-defined locals" or "cdef-locals").
> 2) each function also has a set of "runtime locals" accessible from
> locals().
> 3) exec() can add variables to locals() (runtime) set but it cannot add
> any to cdef-locals.
> 4) locals() may be a superset of cdef-locals (but entries in cdef-locals
> which value is still undefined are not shown in locals())
> 5) due rule 4, exec() cannot add a variable to locals() if it is already
>   present in the in cdef-locals.
> 6) when eval() runs, it uses locals() set for lookup
>
> Perhaps rule 5 is to prevent exec() to modify any arbitrary variable of
> the caller...
>
> Anyways, nice food for our brains.
>
> On Wed, Jul 20, 2022 at 04:56:02PM +0000, george trojan wrote:
> >I wish I could understand the following behaviour:
> >
> >1. This works as I expect it to work:
> >
> >def f():
> >    i = 1
> >    print(locals())
> >    exec('y = i; print(y); print(locals())')
> >    print(locals())
> >    exec('y *= 2')
> >    print('ok:', eval('y'))
> >f()
> >
> >{'i': 1}
> >1
> >{'i': 1, 'y': 1}
> >{'i': 1, 'y': 1}
> >ok: 2
> >
> >2. I can access the value of y with eval() too:
> >
> >def f():
> >    i = 1
> >    print(locals())
> >    exec('y = i; print(y); print(locals())')
> >    print(locals())
> >    u = eval('y')
> >    print(u)
> >f()
> >
> >{'i': 1}
> >1
> >{'i': 1, 'y': 1}
> >{'i': 1, 'y': 1}
> >1
> >
> >3. When I change variable name u -> y, somehow locals() in the body of
> >the function loses an entry:
> >
> >def f():
> >    i = 1
> >    print(locals())
> >    exec('y = i; print(y); print(locals())')
> >    print(locals())
> >    y = eval('y')
> >    print(y)
> >f()
> >
> >{'i': 1}
> >1
> >{'i': 1, 'y': 1}
> >{'i': 1}
> >
>
> >---------------------------------------------------------------------------NameError
> >                                Traceback (most recent call last)
> >Input In [1], in <cell line: 10>()      7     print(y)      8     # y
> >= eval('y')      9     #print('ok:', eval('y'))---> 10 f()
> >
> >Input In [1], in f()      4 exec('y = i; print(y); print(locals())')
> >   5 print(locals())----> 6 y = eval('y')      7 print(y)
> >
> >File <string>:1, in <module>
> >NameError: name 'y' is not defined1.
> >
> >Another thing: within the first exec(), the print order seems
> >reversed. What is going on?
> >
> >BTW, I am using python 3.8.13.
> >
> >George
> >--
> >https://mail.python.org/mailman/listinfo/python-list
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> https://mail.python.org/mailman/listinfo/python-list
>